Arthritogenic alphaviruses, including Ross River virus (RRV) and chikungunya virus (CHIKV), are re-emerging, mosquito-transmitted alphaviruses that cause both endemic and explosive epidemics of debilitating musculoskeletal inflammatory disease. Major outbreaks have occurred in the South Pacific, Africa, and Asia involving millions of persons. Locally-acquired CHIKV disease outbreaks in Europe and the return of RRV and CHIKV-infected travelers to nonendemic regions, including the United States, has highlighted concern that these viruses will continue to spread to new areas. There are no specific therapies for the treatment of alphavirus-induced rheumatological disease and no licensed vaccines. A striking feature of RRV and CHIKV infections is the development of chronic, incapacitating musculoskeletal disease in up to 64% of infected individuals, which has been linked in both humans and animal models to persistent infection in the affected tissues. Thus, understanding the immunological mechanisms that control arthritogenic alphavirus infection is essential for the development of new therapeutic strategies. Utilizing mouse models that recapitulate important aspects of human disease, we detected abundant nitrotyrosine, a marker of peroxynitrite activity, in the musculoskeletal tissues of RRV- and CHIKV-infected mice. Peroxynitrite is a potent inducer of nitrative stress that causes tissue damage and suppresses T cell responses. Arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS; Nos2), enzymes that synergize to produce peroxynitrite, were upregulated in inflammatory lesions and tissue-infiltrating macrophages of RRV- and CHIKV-infected mice, and mice lacking Arg1 in myeloid cells or Nos2 expression had reduced viral loads and tissue damage at late times post-RRV infection, indicating that Arg1 and iNOS thwart host control of RRV infection. Additional preliminary data suggests that myeloid cell Arg1 acts on T cells at the sites of infection to limit virus control. Based on these data, we hypothesize that arthritogenic alphavirus infection induces a localized immunosuppressive environment in musculoskeletal tissues that promotes persistent infection and chronic disease. We propose to define mechanisms by which iNOS and Arg1-mediated production of peroxynitrite suppresses T cell responses, and to define the cytokine signaling pathways that promote the immunosuppressive activity of macrophages, to prevent control of RRV infection in musculoskeletal tissues and promote chronic disease. These studies will define the role of Arg1 and iNOS, and signaling pathways that regulate their expression, in host control of acute and chronic arthritogenic alphavirus infection and tissue injury and will aid the development of new host-targeted therapeutic strategies to treat alphavirus-induced rheumatological disease.